Method and device for marking and method for identifying a document using unique imprint derived from unique marking variations characterized using frequency analysis

ABSTRACT

A process for securing a document including a step of printing a distribution of dots on said document, said printing, as a result of unanticipated unknowns in printing, causing an unpredictable variation, dot by dot, of at least one geometric characteristic of the printed dots and prior to said print step, a step generating said distribution of dots so that at least half the dots of said distribution are not laterally juxtaposed to four other dots of said dot distribution, and at least one dimension of at least one part of the dots of said distribution of dots is of the same order of magnitude as the average for the absolute value of said unpredictable variation. In embodiments, during the step generating the dot distribution, more than half the dots do not touch any other dot of said distribution. In embodiments, during the step of generating the dot distribution, dots are produced of which at least one geometric characteristic is variable, the geometric amplitude of the generated variation having the order of magnitude of the average dimension of at least one part of the dots.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of co-pending application Ser. No. 13/706,646 filed on Dec. 6, 2012; which is a division of application Ser. No. 12/302,917 filed on Aug. 26, 2009; which is the 35 U.S.C. 371 national stage of International application PCT/FR2007/000918 filed on Jun. 1, 2007; which claims priority to FR applications 0604937, 0606588, and 0611402 filed on Jun. 1, 2006, Jul. 19, 2006, and Dec. 26, 2006, respectively. The entire contents of each of the above-identified applications are hereby incorporated by reference.

FIELD OF THE INVENTION

This invention concerns a process and a device for securing documents. It envisages, in particular, identifying a document in a unique way, authenticating it, i.e. being able to detect its copying and/or carrying, on the document, information relative to this document, for example information identifying an owner of intellectual property rights connected to the document and/or its place of manufacture. The term document includes all data carriers, for example hardcopy documents, blueprints, packaging, manufactured items, molded items and cards, e.g. identification cards or bankcards.

BACKGROUND OF THE INVENTION

The different types of document printing are divided into two groups: one known as “static”, in which each document receives the same printed mark, for example an “offset” analog print process, and the second known as “serialized” digital, in which each document receives an individualized item of information, for example an ink-jet print process controlled by an individualization program, and a process for printing a serial number.

For offset printing, which is one of the most commonly used print methods for boxes and packaging, a plate is generated for each color printed in the document, and this plate's content is printed hundreds of thousands, even millions, of times. In this case, the same content, inserted on the printing plate, is printed on every document for every print. Flexography, typography and gravure printing are other examples of what are called static printing methods. In static printing, documents cannot be identified individually, in theory, since the same mark is printed each time. In addition, when the printing is static and makes use of analog processes, it is more difficult to control the exact number of documents printed. The risks of counterfeiting through printing a larger quantity of documents than the owner of the rights has authorized are therefore significant. How can you ensure that the number of prints specified by the manufacturing order, often less than the plate's usage limit, has been respected? How can you ensure that all the unused prints (start or end of the series, faults, order cancelled, etc) and all the plates, films and other objects that allow the documents to be reconstituted never fall into the hands of counterfeiters?

Serialized printing, by allowing each document to be precisely and unequivocally identified, is generally preferable to static printing. In effect, each identifier being only printed once in serialized printing, reading a double means that an alarm can be triggered: a double is an identifier that is identical to a previously read identifier.

In a general way, there are several points to be made secure in order to protect identifier and/or anti-copying marks: the source file, possibly the CAP file that contains it, and, in the case of offset printing, the plates and any films.

It is possible to perform the equivalent of serialized printing of an anti-copying mark on an item already printed statically by, in a second step, printing a unique code or serial number that is uncoded or, preferably, encrypted. This serialized printing can, for example, take the form of a two-dimensional bar code. Outwardly, this procedure makes it possible to track each document individually and at the same time retain a sure way of detecting copies. Stolen documents that have not received the serialized print would not bear a valid identifier.

This approach does not, however, solve all the problems. In effect, while a wrongdoer cannot identify the falsified documents as the printer would have done, the unique code printed by the serialization printer, generally offering a limited print quality, is not protected against copying.

Counterfeiters having in their possession documents to be identified as authentic can therefore copy one or more valid unique codes and re-copy them onto documents to be identified as authentic.

DESCRIPTION OF THE RELATED ART

The prior state of the art contains several methods exploiting measurable physical characteristics in order to characterize and identify each document in a unique way. In general, the measurable physical characteristics chosen are of a random nature, and according to the actual state of the art and technologies cannot be copied, at least not in a cost-effective way. These methods enables all the documents considered “valid” to be controlled: only those documents for which the physical characteristics, comprising a unique set, have been memorized are considered valid.

For example, U.S. Pat. No. 4,423,415 describes a method enabling a sheet of paper to be identified according to its local transparency characteristics. Several other procedures are based on inputting unique and non-reproducible physical attributes of the material in order to generate a unique and non-transferable signature of said document. For example, documents WO 2006 016114 and US 2006/104103 are based on the measurement of the diffraction pattern induced by a laser ray applied to a precise area of the object.

Although they offer an interesting solution to the problems mentioned above, the approaches based on extracting a signature from the material are difficult to use for a number of reasons. Firstly, recording signatures when the documents are produced requires a costly optical reader and is difficult to integrate into production lines. These latter may, moreover, have very high working speeds. In a general way, it seems that these techniques are only applicable to small-scale production. In addition, the reader used for checking, in the field, is also too costly for a number of applications. It is also bulky and not easy to use, while often the checks in the field must be done rapidly and unobtrusively. Finally, it is not possible to extract a unique signature for all materials: glass and objects that are too reflective are excluded, in particular, at least for measurements of a laser's diffraction.

SUMMARY OF THE INVENTION

This invention aims to remedy these inconveniences and, in particular, the difficulties and limitations of applying identification methods based on the unique physical attributes of the document's matter.

The digital authentication codes, also called “DAC” below, are digital images that, once marked on a medium, for example by printing or local modification of the medium, are designed so that some of their characteristics, generally automatically measurable from a captured image, are modified if a marked image is copied. The digital authentication codes are generally based on the degradation of one or more signals sensitive to copying during the copy step, a signal being borne by image elements with measurable characteristics sensitive to copying. Certain types of digital authentication codes can also contain an item of information allowing the document containing it to be identified or tracked.

There are several types of digital authentication codes. The copy detection patterns, also called “CDP” below, are dense images, generally of a pseudo-random nature. Their reading principle is based on an image comparison in order to measure an index of similarity (or dissimilarity) between the original copy detection pattern and the copy detection pattern captured, for example by an image sensor: if this captured image is a copy it will have a lower index of similarity than if it is an original.

Like the two-dimensional bar codes, the secured information matrices, also called “SIM” below, are images designed to carry a large quantity of information in a robust way. However, unlike two-dimensional bar codes, secured information matrices are sensitive to copying. On reading, an error rate is measured for the coded message extracted from the matrix, a rate that is higher for the copies than the originals, which allows these copies to be distinguished from original prints.

Unless marked in a special way, for example with invisible ink, the copy detection patterns and secured information matrices are visible. In addition, marking the copy detection patterns and secured information matrices in an invisible way is not always possible, due to cost or manufacturing constraints. The visibility of an anti-copying mark can be a disadvantage in terms of esthetics and, in certain cases, security since the counterfeiter is informed of their presence.

There are also digital authentication codes that are naturally invisible or at least difficult to see.

For example, some digital marks (known under the name “watermarks”) integrated into printed images are designed so as to be damaged when the printed image is reproduced, for example by photocopying. The measurement of the digital watermark's degree of deterioration, lower in the original print than in a copy of it, makes it possible to detect these copies.

The combination of several watermarks with different degrees of sensitivity to copying makes it possible, by comparing the respective energy levels, to detect the copies. Integrating digital watermarks in the production procedures of documents is, however, more complex, which limits their use: in effect, unlike copy detection patterns and secured information matrices, the digital watermark cannot be simply “added” to the image; the digital watermark is, in fact, a complex function of the message to be added and of the original image, the digital watermark's energy being locally adjusted according to the original image's masking properties. Integrating digital watermarks in documents or products entails sending the source image to a marking/printing central processing unit that integrates the digital watermark and sends back a marked image. This procedure is not very practical, because of the often large size of the files and related image security problems. In contrast, for marking/printing with a copy detection pattern or secured information matrix, the source image does not have to be sent to the marking/printing central processing unit: conversely, it is the image of the copy detection pattern or secured information matrix, generally of a small size, for example several kilobytes, that is sent to the holder of the image files that will be affixed onto the document or product. In addition, it is very difficult to stabilize the reading of digital watermarks, which makes the determination of the copy from the original of a document more random. In effect, the risks of error are generally noticeably higher with digital watermarks than with copy detection patterns and secured information matrices.

There are also asymmetric modulation spatial marking processes, also called “AMSM” below, such as those described in documents WO 2006 087351 and CH 694 233. Just like digital watermarks, AMSMs allow documents to be marked invisibly, or at least unobtrusively. AMSMs are generally patterns of dots, which are added as an additional layer to the document to be marked. For example, in the case of an offset print process, an additional plate bearing only the AMSMs is overprinted on the document. In this way, the AMSMs are more easily integrated than digital watermarks into the document production process, the source image not being required by the marking/printing central processing unit. However, unlike copy detection patterns and secured information matrices, the AMSMs generally require an additional plate and ink, which makes their use more complex and more costly. In addition, just like digital watermarks the AMSM detection methods can be imprecise. In fact, it is known that the marking/printing entails an analog uncertainty concerning the precise positioning of the marked image. This uncertainty, at the level of the dimension of the printed elementary dot, even below this, has a not insignificant effect on the detection of copies when the surface marked has a significant size. However, AMSM detection methods, based on auto-correlation and cross-correlation, cannot take this uncertainty of position into account. This increases the imprecision in reading the mark and, as a consequence, reduces the ability to distinguish between the originals and the copies.

When the capture is done by flat-bed scanners, allowing both a large capture surface and a sufficient capture resolution, the AMSMs enable simple copies to be detected, for example photocopies, even high-quality photocopies done by capture with a high-precision or high-resolution scanner, followed by reprinting. Nevertheless, in the face of a determined counterfeiter, AMSMs offer reduced protection against copying. In effect, after the high-resolution capture the counterfeiter can use manual image processing tools, such as “Photoshop” (registered trademark), possibly combined with automatic image processing tools (such as “Matlab”, registered trademark), in order to restore all the detected dots in their initial form. In the case of a high-quality copy, the dots will no longer be weaker in the copied mark than in the original mark, and the copy has a strong chance of not being detected as a copy. Thus, a determined counterfeiter can generally make an identical copy of the information contained in an AMSM, which means that this method cannot be considered secure in the long term.

The different types of document printing are divided into two groups: one known as “static”, in which each document receives the same printed mark, for example an “offset” analog print process, and the second known as “serialized” digital, in which each document receives an individualized item of information, for example an ink-jet print process controlled by an individualization program, and a process for printing a serial number. For the most commonly used print methods (in particular offset), the AMSMs (and other digital authentication codes) are printed statically. As the types of printing most commonly used for AMSMs and digital authentication codes are static, it is not possible to vary the mark and the contained message on each print.

Nevertheless, it may be desirable to be able to uniquely characterize, and thus identify, each print from a single source image.

To this end, according to a first aspect, the present invention envisages a document identification process, that comprises:

-   -   a step of generating an image,     -   a step of marking a plurality of documents to form said image on         each said document with unique variations on each document, the         majority of the images formed on said documents presenting a         physical anti-copy characteristic satisfying a pre-defined         criterion such that said characteristic of the majority of the         copies that can be produced based on these images do not satisfy         said pre-defined criterion,     -   a step of characterizing said variations to form a unique         imprint of the mark formed, for each said document and     -   a step of memorizing said unique imprint.

Thanks to these provisions, the same mark, which represents the image generated in advance, makes it possible to both detect a copy of an original document and individualize each document among all the originals. This benefit is particularly advantageous since it is not necessary to add an identifier to each document, which is burdensome, in order to recognize it among all the originals. Thus, the present invention makes it possible to generate and use images designed to allow the identification of each document through the unique characteristics of each marking, for example by printing.

According to particular features, said image only comprises two colors.

Thanks to these provisions, as a result of the contrast between the dots representing the image, its surface can be reduced and its processing made simpler, and therefore faster. Also, the image is integrated more easily into the majority of print processes.

According to particular features, frequency analysis is utilized during the step characterizing variations. Thanks to these provisions, the characterization is less sensitive to slight, for example subpixel-level, variations in positions of an image sensor enabling this.

According to particular features, a secret key is utilized during the step characterizing variations. Thanks to these provisions, the characterization is itself protected in its realization principle.

According to particular features, during the memorization step, the imprint is stored in a database. Thanks to these provisions, during the life and movements of the document, an imprint recognition device can make use of the database or, alternatively, images of documents or imprints can be sent to a means of comparison with the imprints stored in this database in order to obtain an identifier of the document.

According to particular features, during the memorization step, the document is marked with a mark representing the imprint affixed individually on the document. Thanks to these provisions, a recognition device does not need a database to identify the document. It is noted that the mark representing the imprint can be a bar code, a two-dimensional bar code or a datamatrix (registered trademark), for example.

According to particular features, said mark representing the imprint is encrypted. According to particular features, the encryption utilized to encrypt the imprint uses a public key. Thanks to each of these provisions, the imprint's content is protected by encryption, this being especially robust when it utilizes a public key.

According to particular features, said image is designed to be degraded during copying of a document obtained by the marking step.

In this way, the same object combines off-line traceability and authentication properties for digital authentication codes with unique identifier pattern characterization properties.

Another objective of the present invention consists of using an identifier pattern's imprint in “identification” mode, in which the imprint generated by capturing the image of an identifier pattern is compared against a set of pre-calculated imprints in order to determine whether it matches one of said pre-calculated imprints.

Another objective of the present invention consists of using the data contained in the digital authentication code used as identifier pattern in order to facilitate the search for the related imprint in a database, the data making it possible to determine a sub-set in which the search will be carried out.

Another objective of the present invention consists of using an identifier pattern's imprint in “check” mode, in which the imprint generated is only compared against to pre-calculated imprint of said document, the latter being, for example, affixed to the document or able to be derived from the document's data (for example, serial number and link to a database).

According to a second aspect, the present invention envisages a document processing process, that comprises:

-   -   a step of capturing an image of a mark printed on said document,     -   a step of extracting a physical anti-copy characteristic of said         image,     -   a step of determining whether said characteristic satisfies a         pre-determined criterion,     -   a step of extracting unique variations of said printed mark by         processing said image,     -   a step of determining an imprint of said variations and     -   a step of determining whether said imprint corresponds to a         memorized imprint.

According to a third aspect, the present invention envisages an object authentication process, characterized in that it comprises:

-   -   a step of affixing a mark to said object in such a way that the         affixed mark presents unpredictable errors due to the physical         characteristics of the means utilized during the marking step,     -   a step of capturing an image of said mark and     -   a step of determining physical characteristics of said         unpredictable errors by processing said image.

Thanks to these provisions, even though the same marking process is utilized, without modification, for example by etching or printing, on many objects, the physical characteristics of the marking errors make it possible to give each mark, and thus each associated object, a unique identification. To some extent, the mark comprises a unique fingerprint. The utilization of the process that is the subject of the present invention is therefore both inexpensive and individualized.

According to particular features, the process as described in brief above comprises, in addition, a step memorizing an item of information representing the physical characteristics of the unpredictable errors. Thanks to these provisions, when a new image capture is carried out with a marked object and new image processing is applied, the result of this image processing can be compared to the memorized information in order to retrieve the object's identification.

According to particular features, during the marking step, a mark is affixed that is formed from a matrix of dots with a resolution such that at least two percent of the mark's dots are erroneous compared to the original dot matrix. Thanks to these provisions, the error quantity is significant and allows the mark and the object to be uniquely identified.

According to particular features, during the marking step, a mark is affixed representing data relating to the object that bears it.

Thanks to these provisions, the reading of the data relating to the object that bears the mark provides an origin and/or means of access to a database of physical characteristics of the errors.

According to particular features, during the marking step the mark affected with the unpredictable errors is affixed with a resolution such that copying it increases by at least fifty percent the level of errors in the copied mark compared to the original mark.

According to particular features, during the image processing step characteristics of the distribution of said errors in said mark are determined, as physical characteristics of the unpredictable errors. Thanks to these provisions, whatever the conditions in which a new image of said mark is captured, the error distribution characteristics can be retrieved.

According to particular features, the authentication process as described in brief above comprises, in addition, a robust marking step during which a robust mark is affixed to said object bearing information relating to the physical characteristics of the unpredictable errors. Because this second mark is robust, it can resist slavish copying and enable the object to be identified.

According to particular features, during the robust marking step, a code key is utilized for the physical characteristics of the unpredictable errors.

According to particular features, said code key is a public key.

Thanks to each of these provisions, the unique identity of the mark, and therefore of the object to which it is affixed, can be determined by utilizing the robust mark.

According to a fourth aspect, the present invention envisages an object authentication device, characterized in that it comprises:

-   -   a means of affixing a mark to said object in such a way that the         affixed mark presents unpredictable errors due to the physical         characteristics of said means of affixing the mark,     -   a means of capturing an image of said mark and     -   a means of determining physical characteristics of said         unpredictable errors by processing said image.

The present invention also aims, through some of its aspects, to remedy at the same time the weaknesses or limits of use for digital watermarks, AMSMs, secured information matrices and copy detection patterns.

To this end, according to a fifth aspect, the present invention envisages a process for securing a document, characterized in that it comprises:

-   -   a step of printing a distribution of dots on said document, said         printing, as a result of unanticipated unknowns in printing,         causing an unpredictable variation, dot by dot, of at least one         geometric characteristic of the printed dots and     -   prior to said print step, a step generating said distribution of         dots so that:         -   at least half the dots of said distribution are not             laterally juxtaposed to four other dots of said dot             distribution, and         -   at least one dimension of at least one part of the dots of             said distribution of dots is of the same order of magnitude             as the average for the absolute value of said unpredictable             variation.

By means of at least this aspect, the present invention thus makes it possible to exploit the individual geometrical characteristics of the marked dots, and to measure the variations in the characteristics of these dots so as to integrate them in a metric (i.e. determine whether they satisfy at least one criterion applied to a measurement) allowing the originals to be distinguished from copies or non-legitimate prints.

According to particular features, during the step generating the dot distribution, more than half the dots do not touch any other dot of said distribution.

Thus, unlike secured information matrices and copy detection patterns, and like AMSMs and digital watermarks, the present invention makes it possible to insert invisible or unobtrusive marks. In addition, these marks are easier to integrate than digital watermarks and AMSMs. They enable more reliable copy detection than digital watermarks and they can be characterized individually in a static print process, which allows the unique identification of each document.

According to particular features, during the step of generating the dot distribution, dots are produced of which at least one geometric characteristic is variable, the geometric amplitude of the generated variation having the order of magnitude of the average dimension of at least one part of the dots.

The present invention thus makes it possible to generate and use, in an optimal way, images of variable characteristic dot patterns, also called “VCDPs” below, designed to make copying by identical reconstitution more difficult, even impossible.

According to particular features, said generated variation corresponds to a variation in the position of dots, in at least one direction, with respect to a position where the centers of the dots are aligned on parallel lines perpendicular to said direction and separated from at least one dimension of said dots in that direction.

The present invention thus makes it possible to exploit the precise position characteristics of the dots, and to measure the very limited variations in the precise position of the dots so as to integrate them in a metric allowing the originals to be distinguished from copies.

According to particular features, said generated variation corresponds to a variation in at least one dimension of dots, in at least one direction with respect to an average dimension of said dots in that direction.

According to particular features, said generated variation corresponds to a variation in shape of the dots with respect to an average shape of said dots in that direction.

According to particular features, during the generation step, said dot distribution represents a coded item of information.

The present invention thus allows information to be stored or carried in the distribution of dots with variable characteristics. For an equal quantity of information content, the dot distributions can cover a significantly smaller surface area than AMSMs, for example several square millimeters, which allows their high-resolution capture by mobile capture tools, and consequently great precision in reading.

According to particular features, the process as described in brief above comprises a step of capturing the image of the printed dot distribution and a step of determining a unique signature of said printed distribution, according to said unpredictable print variation.

The present invention thus makes it possible to use the variations belonging to each print so as to uniquely characterize each of the prints of a single source variable characteristic dot pattern.

According to particular features, the process as described in brief above comprises a step determining a magnitude representing the unpredictable print variation, the step generating the dot distribution being a function of said magnitude.

According to particular features, the process as described in brief above comprises a step of detecting a copy according to the magnitude representing the unpredictable print variation, said copy detection step comprising a step of comparing said representative magnitude against a pre-defined value, and a step of deciding the authenticity of the document according to the result of the comparison.

According to a sixth aspect, this invention envisages a process for securing a document against copying causing, as a result of unanticipated unknowns in copying, a so-called unpredictable “copy” variation, dot by dot, of at least one geometric characteristic of the printed dots, characterized in that it comprises:

-   -   a step of printing a distribution of dots on said document, said         printing causing, as a result of unanticipated unknowns in         printing, a so-called unpredictable “print” variation, dot by         dot, of said geometric characteristic of the printed dots, the         average magnitude of the unpredictable print variation being of         the same order of magnitude as the average minimum magnitude of         the unpredictable variation of said copies and     -   a step of determining a physical magnitude representing the         unpredictable print variation.

It is noted that the step determining the physical magnitude may be preliminary to the print step, as in the process that is the subject of the fifth aspect of the present invention, or after a step printing samples with a view to the characterization of the print step.

According to a seventh aspect, the present invention envisages a device for securing a document, characterized in that it comprises:

-   -   a means of printing a distribution of dots on said document,         said printing, as a result of unanticipated unknowns in         printing, causing an unpredictable variation, dot by dot, of at         least one geometric characteristic of the printed dots and     -   a means of generating said distribution of dots designed, prior         to printing, to generate said dot distribution so that:         -   at least half the dots of said distribution are not             laterally juxtaposed to four other dots of said dot             distribution, and         -   at least one dimension of at least one part of the dots of             said distribution of dots is of the same order of magnitude             as the average for the absolute value of said unpredictable             variation.

According to an eight aspect, the present invention envisages a device for securing a document against copying causing, as a result of unanticipated unknowns in copying, a so-called unpredictable “copy” variation, dot by dot, of at least one geometric characteristic of the printed dots, characterized in that it comprises:

-   -   a means of printing a distribution of dots on said document,         said printing causing, as a result of unanticipated unknowns in         printing, a so-called unpredictable “print” variation, dot by         dot, of said geometric characteristic of the printed dots, the         average magnitude of the unpredictable print variation being of         the same order of magnitude as the average minimum magnitude of         the unpredictable variation of said copies and     -   a means of determining a physical magnitude representing the         unpredictable print variation.

According to a ninth aspect, the present invention envisages a process for reading a distribution of dots on a document, characterized in that it comprises:

-   -   a step of capturing an image of said distribution of dots,     -   a step of determining a physical magnitude representing a         geometric variation of dots of said distribution and     -   a step of determining the authenticity of said dot distribution,         according to said physical magnitude.

According to a tenth aspect, the present invention envisages a device for reading a distribution of dots on a document, characterized in that it comprises:

-   -   a means of capturing an image of said distribution of dots,     -   a means of determining a physical magnitude representing a         geometric variation of dots of said distribution and     -   a means of determining the authenticity of said dot         distribution, according to said physical magnitude.

According to an eleventh aspect, the present invention envisages a program that can be loaded in a computer system, said program containing instructions allowing the process that is the subject of the present invention, as described in brief above, to be utilized.

According to a twelfth aspect, the present invention envisages a data carrier that can be read by a computer or microprocessor, removable or not, holding the instructions of a computer program, characterized in that it allows the process that is the subject of the present invention, as described in brief above, to be utilized.

As the particular characteristics, advantages and aims of this securitization process, these securitization devices, this process and these devices for reading, this computer program and this data carrier are similar to those of the securitization process, as described in brief above, they are not repeated here.

The different aspects of the present invention and their respective fundamental, preferential and/or particular characteristics are destined to be combined into a single process or device for securing a document. In this way, the particular features of each of the aspects of the present invention constitute particular features of the other aspects of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages, aims and characteristics of this invention will become apparent from the description that will follow, made, as an example that is in no way limiting, with reference to the drawings included in an appendix, in which:

FIG. 1 represents a digital mark enlarged by a factor of about 20,

FIG. 2 represents the mark illustrated in FIG. 1, after printing, enlarged,

FIG. 3 represents a photocopy of the printed mark illustrated in FIG. 2, enlarged,

FIG. 4 shows a high-quality copy of the printed mark illustrated in FIG. 2, enlarged,

FIG. 5 represents, enlarged, a VCDP, the variable characteristic being, in this case, a dot height,

FIG. 6 represents an enlargement, by a factor of about 200, of a part of a VCDP of FIG. 5, once printed,

FIG. 7 shows two enlarged prints of a single VCDP having a constant dot size before printing,

FIG. 8 represents, enlarged, a secured information matrix comprising, in its center, a VCDP,

FIG. 9 represents, enlarged, a secured information matrix that is surrounded by a VCDP,

FIG. 10 represents, enlarged, a VCDP the four corners of which consist of a dot surrounded by four dots that are close,

FIG. 11 represents, enlarged, a VCDP with lines of dots on the four sides,

FIG. 12 represents, enlarged, a part of a VCDP in the form of a grid,

FIG. 13 represents the absolute value of the two-dimensional Fourier transform of the VCDP shown in FIG. 12,

FIG. 14 represents, enlarged, a detail of a VCDP representing a coded item of information,

FIG. 15 represents, schematically, a particular embodiment of the device that is the subject of this invention,

FIGS. 16A to 20 represent, in the form of logical diagrams, steps utilized in particular embodiments of the various aspects of the process that is the subject of this invention,

FIG. 21 represents an enlarged part of a high-density VCDP,

FIG. 22 represents an enlarged part of a dot dimension gradient VCDP,

FIG. 23 represents, in the form of a logical diagram, steps utilized in a particular embodiment of the process that is the subject of this invention,

FIG. 24 represents, in an enlarged view, a digital identifier pattern utilized in particular embodiments of the process that is the subject of this invention,

FIG. 25 represents, in an enlarged view, the digital identifier pattern of FIG. 24, once printed on an object, in a first print of a series,

FIG. 26 represents, in an enlarged view, the digital identifier pattern of FIG. 24, once printed on an object, in a second print of a series,

FIG. 27 represents a discrete cosine transform of an image captured from one of the printed identifier patterns represented in FIGS. 25 and 26,

FIGS. 28A to 28C represent, in the form of logical diagrams, steps utilized in particular embodiments of the process that is the subject of this invention and

FIG. 29 represents a distribution of the scores for two groups of identifier patterns utilized in particular embodiments of the process that is the subject of this invention.

Before giving the details of the various particular embodiments of this invention, the definitions that will be used in the description are given below.

-   -   “information matrix”: this is a machine-readable physical         representation of a message, generally affixed on a solid         surface (unlike watermarks or digital watermarks, which modify         the values of the pixels of a design to be printed). The         information matrix definition encompasses, for example, 2D bar         codes, one-dimensional bar codes and other less intrusive means         of representing information, such as “Dataglyphs” (data         marking);     -   “document”: this is any (physical) object whatsoever bearing an         information matrix;     -   “marking” or “printing”: any process by which you go from a         digital image (including an information matrix, a document, etc)         to its representation in the real world, this representation         generally being made on a surface: this includes, in a         non-exclusive way, ink-jet, laser, offset and thermal printing,         and also embossing, laser engraving and hologram generation.         More complex processes are also included, such as molding, in         which the digital image is first engraved in the mold, then         molded in each object. Note that a “molded” image can be         considered to have three dimensions in the physical world even         if its digital representation comprises two dimensions. Note         also that several of the processes mentioned include several         transformations, for example standard offset printing (unlike         “computer-to-plate” offset), including the creation of a film,         said film serving to create a plate, said plate being used in         printing. Other processes also allow an item of information to         be printed in the non-visible domain, either by using         frequencies outside the visible spectrum, or by inscribing the         information inside the surface, etc.     -   “Identifier pattern” or “IP”: an image printed from a (digital)         source image, designed and printed so that each of said source         image's prints can be identified with high probability.     -   “Unique characteristics”: the unique physical attributes of an         identifier pattern, allowing it to be distinguished from any         other print from the same source image.     -   “Imprint”: all the values of the characteristics measured,         allowing an identifier pattern to be represented and compared to         other identifier pattern representations.     -   “capture”: any process by which a digital representation of the         real world is obtained, including the digital representation of         a physical document containing an information matrix,     -   “cell”: this is a regular area, generally rectangular or even         square, of a VCDP in which there is, at most, a pre-defined         number of dots, the pre-defined number generally being equal to         one, other than in indicated variants;     -   “generation pixel”: the smallest area considered for generating         a VCDP,     -   “dot”: a printed elementary area of very small size, possibly         variable, making a contrast with a background, the dot generally         being the representation of one or more generation pixels,     -   “capture pixel” or “image pixel”: an area, the image of which         corresponds to a photosensitive elementary dot, or pixel, of an         image sensor,     -   “order of magnitude”: a physical magnitude A is of the same         order of magnitude as a physical magnitude B if the value of A         is between one tenth and ten times the value of B.

In the embodiments of this invention described below with regard to FIGS. 24 to 29, the following are utilized:

-   -   steps 701 to 703, digitally designing identifier patterns,     -   steps 711 to 715, calculating the identifier pattern's imprint         (according to one of the methods described elsewhere),     -   steps 720 to 726, optimizing the printing of identifier         patterns,     -   steps 731 to 734, storing and representing the imprints or         unique characteristics of documents,     -   steps 741 to 749, identifying the imprint by utilizing a         database,     -   steps 751 to 756, checking the imprint without a database,     -   steps 761 to 763, the combined use of an identifier pattern and         a digital authentication code, and     -   steps 771 to 780, securing a document.

With respect to digitally designing an identifier pattern and determining an identifier pattern's print parameters, at the origin of certain particular characteristics of this invention, it has been discovered that, if you print a single source image of a secured information matrix several times, this will be affected with different errors on each print. The same effect has also been noted for copy detection patterns. More generally, it has been noted that, for any image having a sufficient density, 1) printing the image will result in its degradation, and 2) and this will be affected by a different degradation with each print.

To be more precise, this phenomenon is not limited to digital authentication codes. In effect, whatever the density of a digital image, each of its prints will differ from all the other prints, given the random processes utilized in printing. Solely, for low-density images, the differences will be much less numerous and significant. Therefore a much higher capture resolution is needed in order to capture the differences, which are sometimes minimal. In contrast, for digital authentication codes printed with adequate resolution, you do not need to use a particularly high capture resolution (a 1200 dots per inch scanner is shown to be sufficient). In addition, as the differences are very significant, the extraction of the unique characteristics does not have to be done with very great precision, which is advantageous in terms of the cost and stability of the reading algorithms.

The identifier patterns are images designed and printed so as to maximize the differences between each print of a single source identifier pattern. For preference these images are designed in a pseudo-random way (for example with one or more cryptographic keys), but they can be completely random (the difference being that, in the second case, there is no cryptographic key or the key is not kept). However, it is noted that the original digital identifier pattern can be known without compromising security, in theory: in effect, only the identifier patterns recorded (with their imprint) in the database are legitimate, and in theory it is not possible to control the unanticipated unknowns in printing. Therefore, possession of the original image is does not give the counterfeiter any real benefit, which is another advantage, in terms of security, of identifier patterns.

Since the degradations are random in nature and produce a different result for each print of a single source image, each print of an identifier pattern has unique characteristics that cannot be reproduced or transferred. Thus, each of the prints of a single identifier pattern is different from all the others, and therefore per se has the means for identifying it unequivocally. An identifier pattern's imprint can therefore be calculated and used in different ways in order to increase the security of the document that contains it, especially in identification and check modes.

The identifier patterns can be simple rectangles, possibly enclosed by a border making their detection easier, but can also have a special shape, such as a logo, etc. However, the rectangular shape is shown to have advantages with regard to reading (it can be easily identified) and its compatibility with the normal shapes of digital authentication codes or other codes such as one- or two-dimensional bar codes.

An algorithm for designing an identifier pattern is described below:

-   -   during a step 701, a cryptographic key is received, for example         a 32-byte (256 bits) sequence,     -   during a step 702, by using a recursive encryption or hashing         function, the algorithm being initialized with the cryptographic         key, the required number of random bits are generated. For         example, for a 10,000-pixel black-and-white identifier pattern         (1 bit per pixel), 10,000 bits are needed; 8 times more are         needed for a grey-scale identifier pattern (each scale being         equiprobable). Assuming that the SHA-1 hashing function is used         (256-bit input and output), the function must be called 40 times         (one bit per pixel) or slightly less than 320 times (eight bits         per pixel) in order to obtain the necessary bits (since         40×256>=10,000 or 320×256>=80,000). The reader may draw on the         FIPS (acronym for “Federal Information Processing Standard”) and         AES (acronym for “Advanced Encryption Standard”) standards and     -   during a step 703, the bits are assembled into an image, for         example of 100×100 points, possibly completed by a border.

FIG. 24 shows such an identifier pattern, before printing. FIGS. 25 and 26 show two different prints of the identifier pattern shown in 24.

The functions of a digital authentication code can be combined with those of an identifier pattern, since the design and print characteristics of digital authentication codes are close to those required for the identifier patterns. For example, the design algorithms of the copy detection patterns, which require a cryptographic key, are similar to the algorithm described previously, even though the result sought is very different. As for the design algorithms of the secured information matrices, they require both one or more cryptographic keys and one or more messages. The result, however, is similar, i.e. an image with pseudo-random values.

As will be seen below, it turns out that the ideal print conditions for identifier patterns are close to the ideal print conditions for digital authentication codes. Thus it is possible, with respect to both the design and the printed result, to combine the functions of the digital authentication codes and those of the identifier patterns.

With regard to the methods of extracting and, when checking a document, of comparing the imprint of an identifier pattern, first of all a generic extraction and comparison method is described below, which consists of extracting the values of a set of dots of a captured identifier pattern:

-   -   during a step 711, the position of the identifier pattern in the         image of the printed document is determined. For a         rectangular-shaped identifier pattern, for example, the         positions (height, width) of the four edges of the identifier         pattern can be extracted.     -   during a step 712, for a given number of dots to be extracted,         the position in the image is determined and the value of each of         these dots is extracted. For example, 256 dots horizontally and         256 vertically, for a total of 256², a number of dots         corresponding to 2 raised to a whole power is advantageous if,         for example, a FFT (fast Fourier transform) or a DCT (discrete         cosine transform) is used later. The position of the dots can be         determined by using standard geometric techniques, known from         the prior art: determining the position of reference dots (for         example, the four edges of the identifier pattern if this is         rectangular), then determining the position of the dots by         assuming that the captured image has undergone an affine or         perspective transform, for example. The values are typically,         for example, on a scale of 0 to 255, as well as the captured         image. As the positions can be fractional, the value of the dot         taken can be that “of the nearest neighbor”, a method that is         not very costly but not very precise. Interpolation algorithms,         with a cost that increases with the required precision, can also         be used: bicubic, bilinear, etc, interpolation. The result is a         256×256 matrix of integer (nearest neighbor) or floating-point         (interpolation) values.     -   during a step 713, the discrete cosine transform in two         dimensions of the matrix is calculated. The discrete cosine         transform is advantageous since it makes it possible to         significantly compress the signal energy over a small number of         components.     -   during a step 714, a given number of coefficients are selected,         for example the 10×10 lowest frequency coefficients, and         possibly eliminate the constant coefficient, known under the         name “DC” coefficient at position (0,0)     -   during a step 715, the coefficients are re-sequenced into a         vector, which constitutes the imprint of the secured information         matrix.

It is noted that the method described above does not utilize any secret and, consequently, allows anyone whosoever to calculate the imprint. This can be desirable in certain cases, where it is not considered to pose a security risk. In contrast, in other cases it is desirable for only authorized people to be able to calculate the imprint. To do this, you use a cryptographic key that is kept secret and which makes it possible to determine the coefficients constituting the imprint. This key is only divulged to people or entities authorized to reconstitute the imprint. Techniques from the prior state of the art are available to people in this field for selecting the coefficients from the key, generally utilizing a hashing algorithm or an encryption algorithm.

Two imprints corresponding to separate captures can then be compared in multiple ways so as to obtain a measurement of similarity or, conversely, a measurement of distance. By measuring, for example, a coefficient of correlation between them, a measurement of similarity is obtained, which will be referred to as the “score” subsequently.

To validate this method of extracting unique characteristics, an identifier pattern of 100×100 pixels was generated that was printed 100 times on a 600 dots per inch laser printer. A 1200 dots per inch “flatbed” scanner was used to carry out three captures of each printed identifier pattern. An imprint was then calculated for each of the 300 captures done. A score is then measured for each of the 44,850 pairs of imprints (number calculated as follows: 300*(300−1)/2). These 44,850 imprint pairs are separated into two groups:

-   -   one group A of 600 pairs of imprints corresponding to different         captures of the same printed identifier pattern and     -   one group B of 44,250 pairs of imprints corresponding to         captures of different printed identifier patterns.

The score is between 0.975 and 0.998 for group A, and between 0.693 and 0.945 for group B. FIG. 29 shows the distribution of the scores for group A and group B. On the basis of these scores, no confusion between the pairs of the two groups is possible. Thus, by using the imprint calculation method described above, you can determine without ambiguity which of the 100 prints is the source of the captured image.

An “imprint separation degree” is measured, which consists of calculating the difference of the averages of the scores for groups A and B (here 0.992 and 0.863 respectively) and normalizing it by the standard deviation of the scores of group A, here 0.005. A value of 25.8 is obtained. As will be seen later, this index is useful for determining the print and design parameters giving the best results.

A second method of extracting imprints concerning the secured information matrices is described below. This method applies in particular when the identifier pattern also has the functions of a secured information matrix. It explains how a captured secured information matrix's scrambled message is extracted. This scrambled message has a non-zero error rate and the structure of the errors is used as an imprint.

An advantage of this method is that it makes it possible to use software designed to read secured information matrices. This minimizes the cost of the calculations required.

However, the precise reading of a secured information matrix requires a key serving to generate the blocks to align, if necessary. You do not necessarily want to divulge this key in all cases. In addition, the internal alignment variations specific to each print are as far as possible eliminated. This is not necessarily desirable, since these variations play a part in differentiating the different prints of a secured information matrix.

With respect to the method of determining optimal parameters for generating and printing identifier patterns, an optimal level of degradation exists that enables the various prints of a single source identifier pattern to be separated as easily as possible. Thus, if the level of degradation on printing is very low, for example 1% or 2% (1 or 2% of the identifier pattern's cells or pixels are misread from a perfect capture), the various prints of a single identifier pattern are very close to each other and it is difficult to identify them reliably, unless you have a very precise capture and/or a very precise analysis algorithm. Similarly, when the level of degradation is very high, for example 45% or 50% (45 or 50% of the identifier pattern's cells or pixels are misread from a perfect capture, 50% signifying that there is no statistical correlation between the matrix read and the source matrix), the printed identifier patterns are almost indistinct from each other. In reality, the optimal level of degradation is close to 25%, and if the application conditions allow it, it is preferable to be close to this level. In effect, for 25% degradation, assuming that the print variations and therefore the degradations are by nature probabilistic, for each of the dots of the printed identifier pattern, the probability that it differs from the other printed identifier patterns is maximized.

A possible algorithm for optimizing print parameters is described below:

-   -   during a step 720, the surface area available for the identifier         pattern is received, for example a square measuring ⅙ inch,     -   during a step 721, several digital images of identifier patterns         are generated with different digital sizes, corresponding to         various possible print resolutions, for example one identifier         pattern of 66×66 pixels at 400 dots per inch, one of 100×100         pixels at 600 dots per inch, one of 133×133 pixels at 800 dots         per inch, one of 200×200 pixels at 1200 dots per inch,     -   during a step 722, each one of the identifier patterns with         different digital sizes is printed several times, for example         100 times, with suitable resolution so that the dimensions of         the print correspond to the surface area available.     -   during a step 723, for each type, each one of the printed         identifier patterns is captured several times, for example 3         times,     -   during a step 724, each identifier pattern's imprint is         calculated,     -   during a step 725, the similarity scores are calculated for all         the pairs of captured identifier patterns with the same print         resolution and     -   during a step 726, the method described in the test of the         generic imprint extraction method mentioned above is followed to         measure the “imprint separation degree”, for each of the print         resolutions, and the print resolution giving the maximum value         for this degree is selected.

In a variant, several secured information matrices are printed with different print resolutions, and the print resolution resulting in a 25% error rate, as calculated with one of the algorithms described elsewhere, is determined.

In a variant, the print resolution is selected for which the difference between the lowest value for the score calculated on comparing imprints corresponding the identical prints, and the highest value for the score calculated on comparing imprints corresponding to different prints, is the greatest.

With respect to the method of representing and storing characteristics, it is advantageous to reduce the imprint data volume as far as possible. In the case of identification, this involves comparing an imprint to a very large number of imprints stored in a database, which is very costly. This cost is reduced by reducing the size of the imprints to be compared, especially by avoiding using floating-point numbers.

Consider the case of the generic imprint extraction method. The initial data vector extracted from a captured identifier pattern is the 256×256 matrix of extracted values, and its representation by a discrete cosine transform, after selecting coefficients, has 10×10 values. It is advantageous to represent the matrix of values with one byte per value, i.e. 100 bytes.

During a step 727, at least one object is printed with an identifier pattern to produce a secured document.

On the other hand, the coefficients of the discrete cosine transform can be either positive or negative, and in theory are not limited. In order to represent such values with a fixed amount of information, the values must be quantified so as to be represented in binary values. A possible approach is as follows:

-   -   during a step 731, a minimum value and a maximum value are         determined, in advance, for each coefficient. In general, the         minimum and maximum values have the same absolute value.     -   during a step 732, the number of bits or bytes allowing each         value to be represented is determined and     -   during a normalization step 733, for each coefficient of the         discrete cosine transform, the minimum value is subtracted and         then the remainder is divided by the maximum value,     -   during a step 734, the result is multiplied by the number of         possible values of the quantified data, i.e. 256 if one byte is         available for each value. The integer value of the result is         compared to the original quantified value.

In a variant, the quantification steps are optimized so as to minimize the quantification error.

With respect to the method of identification with database, in the case of identification, an identifier pattern must be compared with each of a database's identifier patterns, in order to determine whether it corresponds to one of the database's identifier patterns, in which case the identifier pattern is considered to be valid, and the associated traceability information can be retrieved. If not, the identifier pattern is considered not valid.

In embodiments, the following steps are utilized:

-   -   during a step 741, the imprint of the identifier pattern         contained in the captured image is determined,     -   during a step 742, the obtained imprint's score, or similarity,         is calculated with each of the imprints stored in the database,     -   during a step 743, the maximum similarity obtained is         determined,     -   during a step 744, if the maximum similarity is above a         threshold value, the identifier pattern is deemed valid and,         during a step 745, the associated traceability information is         retrieved,     -   if not, during a step 746, the identifier pattern is deemed not         valid.

In variants:

-   -   during a step 747, if the identifier pattern also has the         functions of a digital authentication code, the traceability         information is extracted.     -   during a step 748, the traceability information allowing the         search space to be reduced can also come from another source,         for example an associated bar code, information from the         controller, etc and     -   during a step 749, this information is used to reduce the search         space in the database. For example, the manufacturing order         information makes it possible to pre-select imprints to be         compared from the sub-set of imprints corresponding to this         manufacturing order.

With respect to the method of checking without database, this requires the pre-calculated imprint of the identifier pattern to be stored on the document. For example, during the step calculating the imprint of each of the legitimate documents, these can be destined both to be stored in a database and to be stored in a secured way on the document.

The storage of the imprint on the document is for preference done by variable printing, i.e. different for each document, on the fly. The imprint can be stored in a one- or two-dimensional bar code, or in a digital authentication code, depending on the print means, the quality of which can be limited.

It is generally preferable to store the imprint in a secured way, for example by using a cryptographic algorithm equipped with a secret encryption key. In this way you avoid the risk of a counterfeiter using non-legitimate documents without having to be connected to a reference database. The following steps are utilized for this:

-   -   during a step 751, the imprint of the identifier pattern         contained in the captured image is determined,     -   during a step 752, the pre-calculated imprint is received,     -   during a step 753, a score, or a similarity, is calculated by         comparing the imprint obtained with the pre-calculated imprint,     -   during a step 754, if the maximum similarity is above a         threshold value, the identifier pattern is deemed valid,     -   if not, during a step 756, the identifier pattern is deemed         invalid.

With respect to a combined use of an identifier pattern with the functions of a digital authentication code, the prior state of the art methods of uniquely characterizing documents use characteristics that cannot be interpreted without making use of a database. On the other hand, while the identifier patterns can simply be images with no significance, as has been seen, they can also be images comprising other functions. In particular they can be digital authentication codes, in which case they can comprise secured information (one or more keys are required to read them), and/or have authentication properties (to distinguish an original from a copy).

The identifier pattern's imprint can be designed to be sufficiently precise to identify the document, but not sufficiently to not be reproducible. In effect, consider the generic method of determining the imprint, based on 100 low-frequency DCT coefficients, possibly represented with one byte each. Any person whatsoever can, in theory, extract these coefficients, and create an image of the same dimension as an identifier pattern by inversing these coefficients. As is understood, this image is very different from printed identifier patterns. Nevertheless, the score obtained by comparing the imprint calculated from an inversed image capture and the original imprint is 0.952. This score, while less than all the scores obtained from comparing imprints of the same printed identifier patterns, is noticeably greater than the scores obtained from comparing imprints of different printed identifier patterns. There is therefore a risk that a counterfeiter seeks to reproduce the imprint of a legitimate identifier pattern.

A better image capture and/or a finer image capture would make it possible to reduce, even eliminate, the risk of such a falsification working. However, this is not always possible. In that case, if the identifier pattern is also a digital authentication code, it is advantageous to use its authentication properties at the same time by utilizing the following steps:

-   -   during a step 761, the identifier pattern is identified or         checked,     -   during a step 762, the key or keys required to authenticate the         digital authentication code is received and     -   during a step 763, it is determined whether the digital         authentication code is an original or a copy.

The digital authentication codes are usually based on the degradation of one or more physical anti-copy characteristics, which are sensitive to copying during the copy step.

Thus, the digital watermarks have a lower energy level in the copy, or even a different energy level ratio between a watermark not very sensitive to copying and a watermark especially sensitive to copying. Similarly in the case of spatial marking techniques, a lower level of energy, or correlation, is noted for the copies. For the copy detection patterns, based on an image comparison, an index of similarity (or dissimilarity) between the original copy detection pattern and the captured copy detection pattern is calculated; if the latter is a copy, the similarity index will be lower. Finally, for secured information matrices, an error rate is measured for the coded message extracted from the matrix; this error rate will be higher for copies (note that, thanks to the coded message's redundancies, the message sent is generally decodable without error).

It is noted that, for each of these methods, one or more values are measured that are generally continuous, and which do not explicitly specify the nature of the document (original or copy). A pre-defined criterion for distinguishing originals from copies must generally be applied, for example by comparing the obtained value or values against one or more “threshold” values, so as to determine whether the measured value or values correspond to a “copy” or an “original”.

With respect to the embodiments of the process for securing documents based on the identifier patterns, the following steps can be utilized:

-   -   during a step 771, the owner of the rights grants a processor a         license to produce a certain number of documents,     -   during a step 772, the owner of the rights sends the processor         one or more identifier pattern(s), possibly having a digital         authentication code function, in the form of a digital image to         be printed on the documents. The identifier pattern can be part         of a digital document's design, or be sent separately. In a         variant, the processor receives the identifier pattern from a         third party authorized by the rights holder,     -   during the step 773, the processor prints the specified number         of documents, with the specified identifier pattern or patterns         on each document,     -   during a step 774, the specified number of printed documents is         sent to the rights holder. In a variant, the documents are sent         to the assembler authorized by the owner of the rights. In a         variant, the specified number of printed documents is directly         processed by the processor during the step 774, as described in         variants.     -   during a step 775, the owner of the rights/the assembler         assembles the finished product (which can contain several         documents),     -   during a step 776, one or more images of the identifier pattern         or patterns is or are captured. In theory, this process is         performed automatically, for example, the products moving on a         conveyor belt under the lens of an industrial camera. The         industrial camera is triggered automatically or via external         activation coming from a sensor,     -   during a step 777, each captured image of an identifier pattern         is stored in a database, with the associated information         (manufacturing order, date, etc.),     -   during a step 778, in real time or deferred, one or more         imprints are calculated for each valid captured identifier         pattern image,     -   during a step 779, with the possible aim of using the identifier         pattern in check mode (without database connection), one of the         imprints, generally the one that occupies the smallest data         volume, is quantified and/or compressed so as to obtain a         compact representation of it. An information matrix (a         datamatrix, a bar code, a secured information matrix SIM, etc.),         for preference made secure with the help of a key, is generated         containing the representation of the imprint. The information         matrix is printed on the document containing the identifier         pattern and     -   during a step 780, if necessary, the set of calculated imprints         is sent, by secure link, to the central server on which the         inspectors are connected in order to check the validity of the         imprints.

In variants:

-   -   the site where the images of identifier patterns are captured         can be located at the printer or the processor, with the         advantage that it can be integrated into the production, and the         disadvantage that it is in an exposed area. The machine used for         calculating and/or storing imprints can be made secure and/or     -   the site can be located at the third-party authorized by the         owner of the rights, generally the same as the supplier of the         identifier patterns used.

FIG. 23 shows:

-   -   a step 605 of determining a matrix of dots representing an item         of information associated to an object to be authenticated,     -   a step 610 of affixing a mark to said object in such a way that         the affixed mark presents unpredictable errors due to the         physical characteristics of the means utilized during the         marking step,     -   a step 615 of capturing an image of said mark,     -   a step 620 of determining physical characteristics of said         unpredictable errors by processing said image,     -   a step 625 of memorizing an item of information representing the         physical characteristics of the unpredictable errors and     -   a robust marking step 630 during which a robust mark bearing         information relating to the physical characteristics of the         unpredictable errors is affixed to said object.

During the step 605, the information matrix is determined, for example in the form of a matrix of areas, each bearing hundreds of dots and each representing an item of binary information. The item of information associated to the product is, for example, the name of its manufacturer, the product's manufacturing order and/or date of manufacture.

During the step 610 you affix the mark formed of a matrix of dots with a resolution such that at least two percent of the mark's dots are erroneous compared to the original dot matrix. For example, a printer's maximum resolution is used. The effect of this resolution is such that, in particular, copying the object, which entails copying the mark, for example by optical or photographic processes, increases by at least fifty percent the level of errors in the copied mark compared to the original mark.

During the step 620 you determine, as physical characteristics of the unpredictable errors, the characteristics of the distribution of said errors in said mark. For example, you determine the vector going from the center of the mark to the barycenter of the errors borne by the mark, and then you assign a weight to the errors depending on their position and you determine a new vector going from the center of the mark to the barycenter of the errors and so on.

During the step 630, the robust mark is, for example, a one- or two-dimensional bar code or a data matrix, known under the name datamatrix (registered trademark). Because this second mark is robust, it can resist slavish copying and enable the object to be identified. For preference, during the step 630, you utilize a code key, for preference a public code key, of the physical characteristics of the unpredictable errors.

Thanks to the utilization of the present invention, even though the same marking process is utilized, without modification, for example by etching or printing, on many objects, the physical characteristics of the marking errors make it possible to give each mark, and thus each associated object, a unique identification.

When a new image capture is carried out with a marked object and new image processing is applied, the result of this image processing can be compared to the memorized information to retrieve the object's identification.

The error quantity is significant and allows the mark and the object to be uniquely identified.

The reading of the data relating to the object that bears the mark provides an origin and/or means of access to a database of physical characteristics of the errors.

Whatever the conditions in which a new image of said mark is captured, the error distribution characteristics can be retrieved.

For the utilization of certain embodiments of the present invention, the inventor has discovered that certain print characteristics can allow the originals to be distinguished from copies very effectively. In particular, the variation in the dimensions, or “size”, in the precise position or shape of the marked dots can be measured and integrated in a metric allowing the originals to be distinguished from copies. It is noted that the variation in the color level (or grey scale) in the image to be printed amounts, because of the screening, to a variation in shape or dimensions. The digital authentication codes mentioned previously are not designed to measure these characteristics precisely. On the contrary, all digital authentication codes of known types have performances deteriorated by the variations in position due to unanticipated unknowns in printing, variations that are disruptive for the measurements used. At best, methods are used to seek to eliminate them. Moreover, the digital watermarks and AMSMs are designed to make it possible to measure the overall characteristics of the signal (energy, for example), which are not very precise for differentiating between the originals and the copies.

FIG. 1 shows a digital mark 105 comprised of a set of dots 110 with random positions surrounded by a black border 115. It is noted that the dots 110 in this original mark are all of the same size, namely 1 pixel for an image printed at 600 pixels/inch. FIG. 2 shows a print 120 of this digital mark. FIG. 3 shows a photocopy 125 of this mark. It is noticed that, in the photocopy 125, the dots 110 have disappeared. With a simple measurement, such as the number of dots still present in the mark, an image of which is captured by an electronic image sensor, or a degree of correlation with the reference mark, it is easy to distinguish an original 120 from a photocopy 125, or a low-quality copy.

FIG. 4 shows a high-quality copy 130. This copy has been made based on a high-quality capture of an image with a scanner, a capture commonly called a “scan”, by restoring to their original state the dots 110 detected automatically (for example, by using the Matlab software system, registered trademark), given that these latter are black and 1/600^(th) of an inch in size. It is seen that all, otherwise most, of the dots 110 present in the original in FIG. 2 are present in FIG. 4. Any counterfeiter's task is, unfortunately, made easier by the fact that, all the dots originally having the same size, the measurement of the size or grey-scale of the dots does not have to be known and the dots can simply be reconstituted in their original size (which, being fixed, is easy to determine over a large set).

For preference, by utilizing certain aspects of the present invention, simply counting the dots present is not enough to distinguish the original from the copy. A method based on the correlation or the level of energy, as used by the AMSMs, is also ineffective for detecting good-quality copies.

For this purpose, in preferred embodiments, in order to widen the opportunities for using dot patterns, determining a document's authenticity entails paying special attention to the geometric characteristics of the dots, which are studied at the local level, unlike prior state of the art methods. In particular, the exact position, shape and/or size of the dots are used for detecting copies, storing information and/or for uniquely characterizing documents. The VCDPs that are the subject of particular embodiments of the present invention thus present the particularity that the exact position, shape and/or size of the dots are variable. For preference, for generating the dot distribution in this VCDP, dots are produced of which at least one geometric characteristic is variable, the geometric amplitude of the generated variation being of the order of magnitude of the average dimension of at least one part of the dots.

The description that will follow concerns:

-   -   VCDP digital design methods,     -   methods of measuring VCDP geometric characteristics,     -   methods of combining the measured geometric characteristics of         the VCDPs in a metric allowing original VCDPs to be         distinguished from copied VCDPs,     -   methods of optimizing the printing of VCDPs,     -   methods of identifying VCDPs on the basis of their geometric         characteristics,     -   methods of checking VCDPs,     -   methods of storing information in the VCDPs and     -   a process for securing documents.

First of all, a method of generating a variable characteristic dot pattern is described below. In order to generate a VCDP, you determine, beforehand, the print quality of the print system that will be used for printing the VCDP on the document, during a step 300. The print quality represents an unpredictable variation of at least one geometric characteristic of the printed dots, dot by dot, caused by the printing, as a result of unanticipated unknowns in printing.

Then you determine the surface area available for printing this VCDP, the resolution of the print system and the maximum density of the dots wanted, during a step 302. For example, the available size can be about ⅙×⅙ inch, and the density 1/100 (about one out of 100 pixels can be printed). The maximum density depends on the accepted degree of visibility for the VCDP, which is a function of the application conditions (color of the ink, medium, type of printing, appearance of the document, for example). The density can be greater, for example a density of 1/16 or 1/9 is possible, even ¼. For preference the VCDP is generated so that the dots printed do not “touch”.

In certain cases, the size available can be much larger, for example several square inches. However, most of the means of capture, for example cameras comprising an array image sensor, offer a capture surface area that does not allow this area to be covered (flat-bed scanners are not generally available when documents or products must be read “in the field”). In this case, you can “tile” the VCDP, i.e. juxtapose the same VCDP, or juxtapose different VCDPs for security reasons. In the rest of the description, these two types of VCDP juxtaposition, respectively identical or different, are called “tiling”.

In assuming that the capture tool can be applied arbitrarily over the print area, the maximum size of the VCDP in order to ensure that at least one VCDP will be fully contained in the capture surface area is equal to half of the smallest side of the capture surface area. For the example mentioned earlier of a 640×480 CCD operating at 1220 dots/inch (surface area of 1.33 cm by 1 cm), the VCDP should not exceed 0.5 centimeters a side.

The VCDP is subsequently generated in such a way that:

-   -   at least half the dots of said distribution are not laterally         juxtaposed to four other dots of said dot distribution, and     -   at least one dimension of at least one part of the dots of said         distribution of dots is of the same order of magnitude as the         average for the absolute value of said unpredictable variation.

The inventors have, in effect, discovered that the print of the original must present such a ratio of orders of magnitude in order to obtain more effective securization functions (authentication and identification) of the document.

In addition, the inventors have discovered that, in certain embodiments, in order to make a document secure against copies causing, as a result of unanticipated unknowns in copying, a so-called unpredictable “copy” variation, dot by dot, of said geometric characteristic of the printed dots, it is preferable that when printing a distribution of dots on the document, said printing causes, as a result of unanticipated unknowns in printing, a so-called unpredictable “print” variation, dot by dot, of said geometric characteristic of the printed dots, the average magnitude of the unpredictable print variation being of the same order of magnitude as the average minimum magnitude of the unpredictable variation of said copies. For preference, you then perform a step of determining a physical magnitude representing the unpredictable print variation, as described elsewhere with reference to the functions of authenticating and identifying a document.

For example, you can use a VCDP of 200×200 pixels printed at 1200 dots per inch, for a printed surface area of ⅙ inch, the “dots” of which measure 2×2 generation pixels when the average of the absolute value of the unpredictable variation is between 0.2 pixels and 20 pixels. Note that a VCDP of 100×100 pixels printed at 600 dots per inch, with dots of 1×1 pixel, may give a comparable result. Nevertheless, a higher image resolution (for the same size of printed area) allows more flexibility in varying the size and/or the position of the dots, as detailed below.

For preference, dots that are superposed, stuck together, or too close, are avoided. For this purpose, the VCDP is divided into adjacent areas, for example into 10×10 areas of 20×20 pixels each, for a VCDP of 200×200 pixels. By leaving a white border of 1 pixel on each of the edges of each area, an area of 18×18 pixels is available for the dot. There are therefore 17×17=289 possible positions for each dot in the area that is reserved for it (the dots taking 2×2 pixels, their highest and left-most points, for example, can only take 17 lateral positions and 17 longitudinal positions).

For security reasons, it is desirable that the VCDP is of a pseudo-random nature, for example generated from a cryptographic algorithm to which a key is supplied that is kept secret. This key is used as the initialization value of an algorithm generating pseudo-random numbers, which can be retrieved by anyone whatsoever who knows the key, but which are very difficult to find for anyone who does not have the key.

As FIG. 16A shows, in order to generate a VCDP you carry out:

-   -   a step 302 of receiving or determining the surface area         available, and of the resolution of the print system and the         print density,     -   a step 304 of receiving a cryptographic key, for example a         32-byte (256 bits) sequence,     -   a step 306 of generating binary values, for example by using a         recursive encryption or hashing function, the algorithm being         initialized with the cryptographic key. For example, for the         example mentioned above, there are 289 possible positions for         the dot, and therefore 9 bits are required to determine the         position of a dot in the area reserved for it. Thus, 900 bits         are required to determine the positions of 100 dots in their         respective areas. Assuming that the SHA-1 hashing function is         used, with 256-bit input and output, the function must be called         four times in order to obtain the necessary binary data and     -   a step 308 of incorporating a dot in each cell and assembling         cells into an image, in this case 200×200 pixels in size. For         example, during this step 308, successive nine-bit sequences are         used, in order to determine the position of a dot in each cell.         When the value represented by this sequence is greater than 289,         the next sequence is taken. If not, the dot is positioned at the         position identified by the sequence, for example by numbering         the successive positions in each line of possible positions.         Then the cells are juxtaposed, for example successively in each         line of cells.

Following the step 308, the VCDPs are incorporated in the print films and the document is printed, during a step 310.

In variants, each dot can have a variable size. For example, the dots can have a surface area greater or less than 2×2 pixels. Thus, the dots can have several sizes offering the possibility of measuring other geometric characteristics that are difficult for the counterfeiter to reproduce. For example, the dots can have two possible sizes, either 2×2 pixels as given previously, or 3×3 pixels, unequal vertical and horizontal dimensions, for example 2×3 or 3×2, also being possible. Note that, in the case of two square dots, an additional item of binary data is needed to identify the size of the dot, an item of data that is added to the nine items of binary data that identify the position of the dot in the area reserved for it. Thus, ten items of binary data are needed per area, and 1000 items of binary data for the 100 cells.

FIG. 5 shows a VCDP 135 with dots whose dimensions vary pseudo-randomly (dots of 2×2 and 3×3 pixels) and a border 140 surrounding the VCDP 135. FIG. 6 shows a detail of the result 145 of printing the VCDP 135 of FIG. 5.

It is noted that, in variants, a border, in this case 140, or arbitrary shapes are added allowing the VCDP to be localized. For example, synchronization blocks are added on the borders or in the VCDP, in the place of areas containing dots.

With respect to the measurements of a VCDP's position characteristics, the inventor has discovered that, while the dots comprising a VCDP can be determined and reconstituted with quasi-certainty by a counterfeiter, it is very difficult for the latter to be able to reduce the uncertainty concerning the precise position of the dots. In effect, when a VCDP is printed, the dots are not necessarily printed in their exact position: this uncertainty is due to unanticipated unknowns in printing, and it is also caused by passing from digital to analog. In effect, by passing from digital values to analog values during printing, then again to digital values when the image is captured, there is an average uncertainty of around a half-pixel, (print and image capture pixels respectively) in the position of the dots, the second uncertainty being independent of the position uncertainties due to the unanticipated unknowns in printing. Note that, according to the stability of the print means, additional position uncertainties can be added. When a high-quality copy is produced, additional re-print position uncertainties are added to the position uncertainties already present. Thus, the variance between a dot's position in the captured image and this dot's position in the original image is, on average, greater if the captured image is a copy than if it is an original.

An algorithm for measuring a VCDP's geometric position characteristics is described below. An image captured, during a step 320, from a document area containing a VCDP and a cryptographic key are used on input. On output from the steps implementing this algorithm, a vector of the position characteristics of the VCDP's dots is obtained.

-   -   by applying the VCDP design algorithm, the original positions of         each of the dots is determined, during a step 322;     -   during a step 324, the position of a set of position reference         shapes in the captured image is determined, it being understood         that the VCDP itself, or a part of it, can serve as a reference         shape, since it is known. For example, these reference cells can         be indicators of corners, the border of a square. Other known         techniques for determining position can also be used, such as         autocorrelating tiled images.     -   from the reference shapes, an image is reconstructed that is the         same size or a whole multiple of the original size, during a         step 326;     -   for each cell, during a step 328, you determine a search area in         the captured image in which the image of the dot must be located         (For example, if the VCDP is printed at 600 ppi (acronym for         “points per inch”) and is captured at 1200 dpi (acronym for         “dots per inch”, signifying capture pixels per inch), an area of         +/−5 pixels corresponds to an area of +/−2.5 pixels in the         original image). A relatively large search area is necessary         since the initial position of the reference cells can be         imprecise;     -   if the dot is a dark color on a light background, during a step         330 the position in the reconstructed image, or in the captured         image, of the pixel having the minimum luminance value in the         defined area is determined, and if the dot is a light color on a         dark background, during step 330, the position in the         reconstructed image, or in the captured image, of the pixel         having the maximum luminance value in the defined area is         determined. This position of a pixel is considered to be the         position of the center of the dot in the captured image;     -   the distances, in each direction, between the two positions are         measured, during a step 332 and     -   all of the distance measurements are compiled into a vector of         geometric characteristics, during a step 334.

In this way, for a VCDP of 100 cells a vector of size 100×2 is obtained. Because of imprecisions in the position of the reference cells, a systematic bias can exist. For preference, during the step 332, this bias is compensated for by calculating the averages of the horizontal and vertical distances and subtracting this average from the corresponding distances (in effect, a zero average is expected for the imprecisions in position).

In variants:

-   -   other characteristic values of each point are used to determine         its position, For example, the luminance value of the dot's         central pixel, the response value to a filter of dots         corresponding to pixels, etc and/or     -   the positions of dots are determined without reconstructing the         image, taking into account the scale factor in the captured         image, as well as the latter's rotation and translation, in         determining search areas for the precise position of each dot.

With respect to distinguishing, or differentiating between original VCDPs and copied VCDPs utilizing the vector of position characteristics, you can proceed as follows:

-   -   for each dot, you calculate the Euclidean distance between the         position of the dot calculated according to the captured image         and the original position, during a step 340,     -   during a step 342, you calculate the average, or median, of this         distance over all the dots, in order to obtain a measurement of         the average distance,     -   during a step 344, you compare this average distance to a         pre-defined threshold and,     -   during a step 346, you determine whether the VCDP is an original         or a copy, in the following way:         -   if the average distance is below the threshold, the VCDP is             considered to be original,         -   otherwise, it is considered to be a copy.

The following example illustrates the proposed method. The same original VCDP has been printed and then captured three times. The average distances calculated over the vectors of position characteristics for the originals are 0.454, 0.514 and 0.503 image pixels. Three high-quality copies have been made, each from one of the three printed VCDPs. The average distances calculated over the vectors of position characteristics for these copies are 0.965, 1.088 and 0.929 image pixels. It is noted that, based on the average distance, the original VCDPs can easily be separated from the copied VCDPs simply by thresholding. Several threshold values are possible, depending on the relative cost of possible errors (“false positive”: detecting a copy as an original, or “false positive”: detecting an original as a copy). A threshold of 0.75 (image) pixels can be an acceptable compromise if the relative costs of each type of error are equivalent.

Other known mathematical techniques, for example based on statistical and/or shape recognition methods, can be used so as to distinguish original VCDPs from copied VCDPs.

With respect to distinguishing, or differentiating between original VCDPs and copied VCDPs using the values of the geometric characteristics of the dots, as was noted above, if the dots are of a constant size it is easy for the counterfeiter to reproduce them with a size that conforms, even if the dots can appear with a variable size in the original mark. In an embodiment, during the generation of a VCDP, one or two dimensions of the dots are made to vary.

During the analysis of a document's authenticity, after having captured an image of the VCDP, during a step 350, you determine the dimension or dimensions of the dots according to their central image pixel's degree of luminance, their response to at least one matrix filter corresponding to image pixels, etc, during a step 352.

Then, the original VCDPs are distinguished from the copies according to the degree of similarity between the dimensions of the original digital VCDP's dots and the dimensions of the corresponding dots in the captured image of the VCDP to be authenticated. For example, you proceed as follows:

-   -   during a step 354, by applying the VCDP design algorithm you         determine a vector of expected dimension characteristics. For         example, the vector of expected characteristics can be the value         of the surface area of the dots or their two dimensions,         horizontal and vertical;     -   during a step 356, you calculate an index of similarity, for         example a coefficient of correlation, between the vector of         expected characteristics and the vector of characteristics         obtained after processing the VCDP's captured image and     -   during a step 358, you determine whether the VCDP is authentic,         by comparing the index of similarity with a pre-defined         threshold value:         -   if the value of the index is greater than the threshold, the             VCDP is considered to be original and         -   if not it is considered to be a copy.

The following example illustrates the proposed method. The same original VCDP, illustrated in FIG. 5, in which the dimensions of the dots vary between 2×2 and 3×3 pixels, has been printed and then captured three times. The vector of characteristics comprises surface area values of 4 and 9 pixels for dot sizes of 2×2 and 3×3 pixels. The vectors of characteristics contain the average luminance value of a region surrounding the dot, less the luminance value of the dot. Therefore there is a higher value if the dot is printed more heavily, which is generally the case for the dots of 3×3 pixels.

The indices of similarity calculated are, for the three original prints, 0.654, 0.673 and 0.701. Then three high-quality copies have been made, each from one of the three printed VCDPs. To make the copies, the positions of the dots were determined, then their degree of luminance was measured. The median degree of luminance of the VCDP's dots has been calculated, and the dots having a luminance less than the median degree of luminance have been considered to be originally of size 3×3 pixels, versus a size of 2×2 pixels for the dots having a degree of luminance greater than the median degree of luminance. The copies have been printed and captured. The indices of similarity calculated are, for the three copies, 0.451, 0.423 and 0.446. It is noted that, based on the average distance, the original VCDPs can easily be separated from the copied VCDPs simply by thresholding. Several threshold values are possible, depending on the relative cost of possible errors. A threshold of 0.55 for the index of similarity can be an acceptable compromise if the relative costs of each type of error are equivalent.

Other known mathematical techniques, for example based on statistical and/or shape recognition methods, can be used so as to distinguish original VCDPs from copied VCDPs.

The description given above basically concerns making a document secure against copying. The rest of the description involves two other forms of securing a document, firstly to uniquely identify documents that have not been printed by a “variable” print process and secondly to carry an item of information concerning the document, for example a reference number, its date of manufacture, its place of manufacture and its manufacturing order, the name of the owner of the intellectual property rights linked to the document or its destination.

Methods of identifying VCDPs on the basis of their geometric characteristics are described below. It entails, in this case, using measured characteristics of the VCDPs to uniquely identify each of the prints from a single source digital VCDP image. In effect, each print of a VCDP produces unique unanticipated unknowns in printing, which can be found in different captures of the same print. Thus, by storing the characteristics of a VCDP's successive prints in a database, or by storing them, preferably in a secure way, on the document containing the VCDP (for example in a 2D bar code), a print of a VCDP, and thus a printed document bearing it, can subsequently be identified, i.e. uniquely recognized, by searching for the correspondence between the geometric characteristics of the VCDP whose image was captured and the stored geometric characteristics.

For preference, the identification and authentication are combined, the same device for capturing and processing the image providing both an indication of the document's authenticity and of the document's identification.

Several geometric characteristics of the dots can be used, such as the precise position, or the measurement of luminance, the dimension or dimensions of the dots and their shape. The degree of luminance, measured by the dot's average, central or minimum grey scale, is especially discriminating since it varies significantly and unpredictably over different prints of the same source image. It is noted that it is not necessary to use dots with variable sizes or shapes in the source VCDP for the characteristics of the dots to vary from one print to the next. In order to illustrate this, FIG. 7 shows two prints of a single VCDP having a constant dot size: a dot 151 is printed more heavily in the lower image than in the upper image, while a dot 152 is printed more heavily in the upper image than in the lower image.

By capturing each of the three printed VCDPs three times, a total of nine captured images is obtained. The vector of characteristics containing the minimum luminance value of the dots is calculated for each of the nine image captures. Then you calculate an index of similarity, i.e. a coefficient of correlation, between the vectors of characteristics of each of the 9*8/2=36 possible pairs of captured images. Out of these 36 pairs, 9 correspond to different captures of the same print, and 25 to captures of different prints. The average for the index of similarity is 0.9566 with a standard deviation of 0.0073 and a minimum value of 0.9474 for the first group, and 0.6203 with a standard deviation of 0.0272 and a maximum value of 0.6679 for the second group. The difference in the index of similarity between the two groups is very significant, and shows that a printed VCDP can be identified without ambiguity on the basis of a vector of characteristics of dots.

FIG. 18 details steps in an identification procedure corresponding to this approach. During a step 402, an image of a printed VCDP is captured. Then, during a step 404, the vector of characteristics containing the average values of the minimum luminance of the dots is calculated. This vector of characteristics, or “signature” of the printed VCDP, contains, for each dot, the average luminance measurement and, possibly, the standard deviation between luminance measurements. It is noted that certain measurements of luminance can be excluded based on their difference from the average of the other measurements and the standard deviation between the other measurements. Then, during a step 406, the vector of characteristics is stored, in a database, with indications concerning the document's production and/or circulation.

During an identification attempt, during a step 410, an image of a printed VCDP is captured. Then, during a step 412, the vector of characteristics corresponding to the stored vector of characteristics is calculated. During a step 414, the stored vector of characteristics closest to the vector of characteristics calculated during the step 412 is determined and the associated information is retrieved.

In a variant, the vector of characteristics determined during the step 404 is also stored on the document itself, in a robust way, i.e. resistant to copying, for example in a two-dimensional bar code or a Datamatrix (registered trademark), preferably encrypted for security reasons. In this case, the document can be authenticated by comparing an index of similarity between the two vectors of characteristics and a threshold value, pre-defined or itself stored in the bar code, during a step 416.

In order to store information in the VCDP, you can, for example, define two possible shapes, two positions or two dimensions for each of the dots, inside the cell assigned to it, so as to store one bit per area. A bit value (“0” or “1”) is assigned to each position, dimension or shape.

With reference to FIG. 5, which illustrates a VCDP with two sizes of dots, the small-size dots (2×2 pixels) can, for example, represent bit value “0”, and the large-size dots (3×3 pixels) can represent bit value “1”.

Thus, for a VCDP with 100 cells, 100 bits can be stored without redundancy. In order to detect and/or correct errors, use of an error-detecting and/or error-correcting code is desirable.

For the case in which the position is used to represent a binary value, it is preferable that the positions corresponding to each of the two values are separated from each other. A possible method for ensuring the separation of the two positions consists of dividing a cell into two equal-sized parts corresponding to the two bit values, and assigning a position pseudo-randomly in the area corresponding to the bit to be coded. It is noted that a dot's position in a cell can represent more than one binary value, because of the multiplicity of possible positions. For example, as was seen above, this position can represent 8 bits over 289 different positions, or 6 bits if one position out of two in each direction is excluded, so as to limit the risk of error in interpreting the position during reading.

On reading the VCDP, you determine a search area around a dot's two possible positions for each sub-cell. In order to determine which of the two sub-cells contains the dot, you determine the minimum luminance value for each of the two sub-cells: the area having the lowest luminance value is considered to be the one in which the dot has been inserted. In a variant, a weight can be assigned to each bit value, according to the difference or ratio of luminance between each of the two sub-cells.

In variants:

-   -   the dot's presence or absence in the cell is used to represent         an information bit (used below in the “grids”);     -   more than one binary value is represented through more than two         possible positions of a dot per cell;     -   more than one binary value is represented through more than two         possible dimensions of a dot per cell;     -   more than one binary value is represented through more than two         possible shapes of a dot per cell and/or     -   the message is encrypted before being encoded.

With respect to integration with other digital authentication codes, the VCDPs can be integrated with digital authentication codes so as to offer an additional layer of protection and/or an unobtrusive means of tracking documents. FIG. 8 shows a secured information matrix 155, which comprises, in its center, an area is which a VCDP 156 is inserted. FIG. 9 shows a secured information matrix 160, which is surrounded by a VCDP 161. It is noted that, in this latter case, the elements allowing the digital authentication code 160 to be located, for example its corners, can be used to locate and determine the approximate positions of the dots of VCDP 161.

In embodiments, means of identifying the VCDP through unobtrusive marks are utilized. In effect, in certain cases it can be desirable for the identifying marks to be more unobtrusive than a border, so that the position, even the presence, of a VCDP can be difficult to detect: for example, limited or broken border marks or corner marks can be inserted, or a digital authentication code or other associated symbols can be used to identify it.

If the same dot pattern is repeated several times, for example by tiling, dots can be identified and located with auto-correlation and cross-correlation techniques, such as the technique described in M. Kutter's article, “Watermarking resisting to translation, rotation and scaling”, Proc. of SPIE: Multimedia systems and applications, Volume 3528, pp. 423-431, Boston, USA, November, 1998.

Another way of introducing unobtrusive reference marks into the VCDP consists of inserting cells constituted of a set of dots with easily identifiable shape characteristics. For example, if you want a dot to serve as a reference, you insert a significant number of dots neighboring a reference dot in order to obtain an easily identifiable cluster of dots. FIG. 10 illustrates a VCDP 165 the four corners 166 of which consist of a cell comprising a central dot and four very close neighboring dots, forming the corners of a square centered on the central dot. For detection, you start by detecting all the dots over a sufficient surface area, which will serve as “candidates”. Then, for each dot, you determine the number of its neighbors at a distance less than or equal to a pre-defined distance. This can be done rapidly if the candidate dots are arranged on a grid, which allows the number of neighbors in a window to be counted rapidly. You retain a limited number of candidates, for example six candidates, that have the greatest number of neighbors. Known geometric techniques can then be used in order to determine which are the candidates corresponding to the reference dots, in this case the corners of the VCDP. For the VCDP 165, you know, for example, that three valid candidates must form a right-angles isosceles triangle.

Another way of introducing unobtrusive reference marks consists of inserting dots based on a line. FIG. 11 illustrates a VCDP 170 with, on the edges, lines 171 bearing a larger number of dots than the parallel dots located inside the VCDP 170. These edge lines can be detected by different line detection algorithms, for example by applying the Hough transform, and/or by applying a Sobel filter allowing the noise to be filtered.

In a variant, you apply tiling of the same VCDP or different VCDPs comprising lines of dots or identifiable marks, for example clusters of dots as illustrated in FIG. 10.

In a preferential embodiment, a VCDP is arranged in the form of a regular grid. In effect, in certain cases it can be advantageous to duplicate the VCDP, by tiling, over a large surface area, even over the whole of the document to be protected. In this way you make it very difficult, even impossible, to destroy a VCDP and you increase flexibility concerning the image capture position. In particular, the same VCDP can be inserted several times by tiling. Equally, a VCDP at least partially different from all the other VCDPs can be inserted. The means of identification described above can be used so as to be correctly positioned to read the VCDP. However, in practice, the reference, synchronization or identification elements can be difficult to detect correctly.

As will be seen below, you can make detection easier by arranging the dots in the form of a grid. The dots are inserted at regular intervals, for example with a space of between 4 and 12 pixels in each direction. On the basis of this principle, there are several ways of representing the information:

-   -   the presence or absence of a dot allows an information bit to be         represented, as in VCDP 175 illustrated in FIG. 12, in which the         presence of a dot corresponds to bit value ‘1’, and its absence         to bit value ‘0’;     -   the size, the shape or an offset of a magnitude less than at         least one dimension of the VCDP's dots allows information to be         represented. For example, the selection of the dot, from four         shapes or four dimension selections, makes it possible to         represent, at each dot of a VCDP 180, two information bits, as         illustrated in FIG. 14, which represents, enlarged, a detail of         the VCDP 180. It is noted that this VCDP's dots can take         dimensions, in pixels (the first figure indicates the height and         the second the width), of 1×1, 2×2, 1×2 and 2×1 pixels         corresponding respectively to the bit values “00”, “01”, “10”         and “11”. Many other combinations and shapes of dots are, of         course, possible.

In a variant, on the principle of a perfectly regular grid, a slight displacement of a dot allows information to be represented. For example, displacing a dot making a surface area of at least two pixels, displacing a pixel horizontally and/or vertically allows two information bits to be represented. Many other possibilities are, of course, possible. Note that such a displacement of dots does not significantly modify the geometric characteristics, and therefore the advantages, of using a grid, especially in terms of identification.

A grid lends itself particularly well to determining the angle of rotation and resizing factor applied to the captured image. In effect, in particular you can make use of a Hough transform of the image, or make use of the determination of energy peaks in the Fourier space. FIG. 13 is a representation of the absolute value of the two-dimensional Fourier transform of the grid of FIG. 12, in which the light value dots correspond to energy peaks. The detection of these energy peaks enables the person skilled in the art to calculate the image's resizing factor and angle of rotation, allowing the latter to receive normalized dimensions, with a view to their processing.

Once the image's rotation and scale are known and, possibly, corrected, you determine the translation value, i.e. the displacement to be applied to the image so as to align the dots of the grid correctly. There are various possible methods for this. Common to all of them is fixing the values of a set of the grid's dots, which are, subsequently, looked for so as to align the grid. For example, the values of a set of dots, chosen pseudo-randomly according to a key, can be fixed. A cross-correlation between the grid's captured and corrected image and an image generated from values of known dots, generates a peak of correlation at the position corresponding to the displacement of the grid.

With regard to the writing algorithm, a large number of methods known to the person skilled in the art are possible. As an example, assume that there is a grid, tiled or not, of 20×20 cells based on the following hypotheses: printing is done at 600 dots per inch, and 1% of the surface area can be marked (to minimize the visual impact of the marking), which makes, on average, one dot every 10 pixels, in each direction. The tile is therefore originally 200×200 pixels; the means of image capture produces images of 640×480 pixels at the capture resolution of 720 pixels per inch. It is noted that you are assured that at least one tile will be fully contained in the captured image.

As input, a message, for example of 8 bytes, a cryptographic key and a scrambling key (the two keys may be identical) are received during a step 502. The message is encrypted during a step 504. Optionally, error-detecting bits can be added to it, for example two bytes making it possible to reduce the risk of error decoding the message by a factor of 2 to the power 16, during a step 506. From the encrypted message concatenated with the error-detecting code, 10 bytes in our example, the message robust to errors is calculated, for example by applying a convolutional code, during a step 508. For a convolutional code of rate two with a memory of seven, for eight bytes on input, a code taking 142 bits is obtained. If you have 20×20 dots=400 positions, this message can be replicated two times, thus obtaining a replicated message of 284 bits, during a step 510. Thus, you have 400−284=116 unused positions, which will be used for storing the synchronization bits used on detection for aligning the tile, as described below. The replicated message is scrambled during a step 512, i.e. in sequence, swapped and transformed by an exclusive-OR function. The swap and the bit values used in the exclusive-OR function are calculated from the scrambling key. In this way, 284 scrambled bits are obtained.

The 116 synchronization bits are generated pseudo-randomly from a key, and their position can also be determined pseudo-randomly, so that they are uniformly distributed in the tile, during a step 514.

The VCDP's image is simply modulated by adding to the positions defined a dot for the bit ‘1’ (there is no modification for the bit ‘0’). Clearly, the dot can be composed to have a variable position, shape and/or one or two dimensions, according to the methods seen previously.

If you want to cover a large surface area, the tiles are added one after another, during a step 516. You can then, according to the variants, always use the same tile or make the message change for each tile. In an example of this second variant, one part of the message can remain fixed, while another part, for example a byte, is randomly determined for each tile. You can also apply a random rotation, a multiple of 90 degrees, to each tile, so as to make a counterfeiter's attempts to analyze the code more difficult. In addition, you can randomly insert synchronization bits or their inverse, i.e. for the synchronization bits you inverse the positions where a dot is inserted. The advantage of this latter approach is that the number of possible configurations increases without the reading becoming more complex, as will be seen. In considering the variations of orientation, you can therefore have 8 possible configurations for the synchronization bits, which makes their analysis more complex in the context of an attack by a counterfeiter.

The 200×200 grid of our example can be replicated, as described above.

The VCDP is then inserted into the print films and the document is printed, during a step 518.

With regard to the reading algorithm, you carry out:

-   -   a step 548 of capturing an image of the document,     -   a pre-processing step 550: pre-processing the image can be         advantageous, especially for the following step of determining         candidate dots. You want, through the pre-processing, to remove         the spurious noises and illumination deviations. The application         of an omni-directional high-pass filter, whose result is         weighted with the initial image, makes it possible, for example,         to reduce the illumination deviations, and the application of a         median filter makes it possible to reduce the noise of isolated         pixels;     -   the candidate dots are determined, during a step 552: the         candidate dots correspond to image pixels whose luminance is a         value below a threshold. This threshold is, for example, a         percentage of the histogram, such as 1%, so that at most 1% of         the pixels are candidate dots. The candidates that are too close         (for example, a distance of less than five pixels) are         eliminated, so that only those having the lowest value in the         region are kept;     -   during a step 554, the vectors of neighboring candidate dots are         determined and the angle of rotation and scale factor are         estimated: a limit value is given for the distance between         neighbors, and all the pairs of dots having a distance less than         this threshold are listed. It this threshold is low enough only         the four direct neighbors of a dot may be associated in a         vector, otherwise the indirect neighbors (diagonally) may be         associated. It is preferable to avoid having non-neighboring         dots associated. To do this, you avoid a threshold value that is         too high. You can then estimate the angle of rotation by         bringing the angle of each of the vectors to a value between 0         and 90 degrees;     -   if the indirect neighbors are included, during a step 556, the         vectors are separated into two groups according to their size         (which is greater by a factor of the square root of two for the         indirect neighbors), and you subtract 45 degrees from the angle         calculated for the indirect neighbors. The scale factor can also         be estimated by measuring the average distance between dots of a         single group, divided by the distance in the original image if         that is known;     -   in an option, during a step 558, the image is restored to form         an image without rotation, in its original size or a whole         multiple of its original size;     -   during a step 560, a matrix is extracted representing the values         represented by the dots: the average distance between the dots,         for example 10 pixels, and the dimension of the reconstructed         image, for example 500×500 pixels, are known. A table with 50         lines×50 columns is therefore generated that will be used to         store the message's estimated values, given that the         relationship between the dimensions of the reconstructed image         and the estimated distance between the dots correspond to a         maximum threshold over the number of dots present in the image.         In reality, if the grid of dots in the captured image had a         significant angle of rotation, the number of dots in the         reconstituted image will probably be significantly lower.     -   in order to fill this table with the estimated values of the         message, during a step 562 you search for a starting dot for         scanning the image. This dot can be, for example, the first         candidate dot detected in the top left of the image, or the         candidate dot with the highest probability of being a dot (for         example the dot with the lowest grey scale). It is noted that it         is important not to make a mistake in the dot selection; an         error can have unfortunate consequences for the rest of the         calculations. You can proceed by iteration on the chosen         starting dot if the following steps of reading the message are         unsuccessful. A value is stored in the table for the selected         dot, for example its grey scale, or its lowest grey scale value,         in a certain area around the central position, so as to avoid an         erroneous measurement if the dot's estimated position is         slightly offset with respect to the actual position, this offset         being due to pseudo-random offsets intended to detect the         presence of a copy or being due to any other imprecision in         positioning. The value is stored in the corresponding position         in the table, the positions of which in our examples go from         (0,0) to (49,49): for example in position (0,0) of the starting         dot is the first dot in the top left, or in position (32,20) if         the starting dot with the highest probability is at position         (322,204). All the image's positions from the starting dot are         then scanned, storing the value found for each dot in the         corresponding position in the table;     -   during a step 564 the grid is aligned: in general, the value         table is offset with respect to the start of the tile. To negate         this offset, you use the known bit values, i.e. the         synchronization bits, which enable the offset to be determined.         Thus, the known synchronization bits can be correlated with the         table of values, for each possible offset, and for the four         possible general orientations (0, 90, 180 or 270 degrees). The         largest correlation value determines the offset and the general         orientation. Alternatively, this could be the lowest or the         absolute correlation value, if a tile is printed in negative,         with respect to another tile. In the case in which the         synchronization bits or their inverse have apparently been         inserted randomly, the highest absolute correlation value is         used to determine the offset. The correlation can be performed         in the Fourier domain in order to reduce the quantity of         calculations. It is noted that the tiles can also be delimited         by continuous lines or by special concentrations of dots, which         can serve as a guide for alignment;     -   during a step 566 you reconstruct the scrambled message: the         scrambled message can then be reconstructed. For example, if         this is contained in a 20×20 grid, a 20×20 matrix is generated         and the values found are inserted into it. The rest of the         decoding of the message can be carried out using standard prior         state of the art methods. Once the scrambled message has been         calculated, the inverse of the operations described in the         reading algorithm described above are applied.     -   during a step 568, optional, like other measurements, if the         grid has special characteristics allowing copies to be detected,         for example the precise position or the size of the dots, these         characteristics can be measured over the determined grid so as         to make a decision about the nature of the document (original or         copy) or the unique characterization/identification of the         document.

FIG. 21 represents an enlarged part of a high-density VCDP, each line of a matrix of dots making up this VCDP noticeably bearing as many black dots as white background, these representing, or not, coded information. In the upper line 185 the lateral position of each dot is variable, whereas, in the lower line 186, the dimensions of the dots are variable, in this case between two values corresponding to 3×3 generation pixels and 2×2 generation pixels. It is understood that such VCDPs present an advantage of compactness in inserting a given number of dots in a document while benefitting from the advantages of the variation in dimension(s), position and/or shape, the average magnitude of which is of the order of magnitude of at least one dimension of a part of the dots and, for preference, less than this dimension. As is easily understood, at least the half of this VCDP's dots are not juxtaposed to four other dots. In contrast, less than half of the dots do not touch another dot.

FIG. 22 represents an enlarged part of a dot dimension gradient VCDP 190. This part corresponds to a corner of a VCDP in which, through successive rings, here the thickness of one line but, in practice, of several lines, the dimensions of the dots are reduced. For example, the dots' dimensions are 6×6 pixels for the bordering ring at the bottom right of the part represented in FIG. 22, then 5×5 pixels for the next ring, then 4×4 pixels and so on.

Thanks to this particular disposition, for at least one of the rings the average magnitude of the unpredictable variations, dot by dot, of at least one geometric characteristic of the dots is of the same order of magnitude as one dimension of the dots of this ring.

It is understood that such VCDPs present an advantage of compactness in inserting a given number of dots in a document while benefitting from the advantages of the variation in dimension(s), position and/or shape, the average magnitude of which is of the order of magnitude of at least one dimension of a part of the dots and, for preference, less than this dimension.

FIG. 15 illustrates a particular embodiment of the device that is the subject of this invention. This device 201, for example a micro-computer and its various peripherals, comprises a communications interface 218 linked to a communications network 202 able to transmit and receive digital data. The device 201 also comprises a means of storage 214 such as, for example, a hard disk. It also comprises a floppy-disk reader 215. The floppy disk 224 can contain data to be processed or being processed as well as the code of a program implementing the present invention, code that, once read by the device 101, is stored on the hard disk 114. According to a variant, the program enabling the device to utilize the present invention is stored in read-only memory 110 (called ROM, acronym for “read-only memory”). In a second variant, the program may be received in order to be stored in the same way as that described above by means of the communications network 202.

The device 201 has a screen 212 making it possible to view the processing results and interact with the device, for example by means of graphical interfaces. By means of the keyboard 213, the user can supply data, surface areas, densities, resolutions, values of parameters or keys, or make implementation choices. The central processing unit 211 (called “CPU”, acronym for “central processing unit”, on the drawing) executes the instructions relating to the utilization of the invention, instructions stored in the read-only memory 210 or in the other storage elements. During powering up, the programs relating to the utilization of the device that is the subject of this invention stored in non-volatile memory, for example ROM 210, are transferred into the random-access memory RAM 217, which then contains the executable code of the program that is the subject of this invention and the registers for memorizing the variables required for utilizing the invention. Obviously, the floppy disks 224 can be replaced by any data carrier such as a compact disk or a memory card. More generally, a means of storing information, readable by a computer or a microprocessor, integrated or not to the device, possible removable, memorizes a program utilizing the process that is the subject of this invention. The communications bus 221 enables communication between the various elements included in the micro-computer 201 or linked to it. The representation of the bus 221 is not limiting and, in particular, the central processing unit 211 is capable of communicating instructions to any element of the micro-computer 201 directly or by means of another element of the micro-computer 201. 

We claim:
 1. A document identification method, comprising: a step of generating an image comprising a matrix of black dots separated by a plain other color, said matrix not being readable with the naked eyes; a step of marking a plurality of documents to form said image on each document of said plurality of document with unique variations on each document, the majority of the images formed on said documents presenting a physical anti-copy characteristic satisfying a pre-defined criterion such that said characteristic of the majority of the copies that can be produced based on these images do not satisfy said predefined criterion; for each document of said plurality of documents, a step of characterizing said variations using frequency analysis to determine a unique imprint representative of a frequency spectrum of the image marked on said document; and a step of memorizing said unique imprint.
 2. A method according to claim 1, wherein, during the step of generating an image, said image is designed to be degraded during copying of a document obtained by the marking step.
 3. The method according to claim 1, wherein, during the step of characterizing variations, encryption with a secret key is utilized.
 4. The method according to claim 1, wherein, during the step of memorizing, the imprint is stored in a database.
 5. The method according to claim 1, wherein, during the step of memorizing, the document is marked with a mark representing the imprint affixed individually on the document.
 6. The method according to claim 5, wherein said mark representing the imprint is encrypted.
 7. The method according to claim 6, wherein the encryption utilized to encrypt the imprint uses a public key.
 8. A document processing method, comprising: a step of capturing an image of a mark printed on said document; a step of extracting a physical anti-copy characteristic of said image; a step of determining whether said characteristic satisfies a pre-determined criterion; a step of extracting unique variations of said printed mark by processing said image; a step of determining an imprint of said variations based on frequency analysis of at least a portion of said image; and a step of determining whether said imprint corresponds to a memorized imprint.
 9. The method according to claim 8, wherein, during the step of determining whether said imprint corresponds to a memorized imprint, a mark representing the imprint affixed individually on the document is read.
 10. The method according to claim 9, wherein said mark representing the imprint is decrypted.
 11. A document identification device, comprising: a means of generating an image comprising a matrix of black dots separated by a plain other color, said matrix not being readable with the naked eyes; a means of marking a plurality of documents to form said image on each document of said plurality of document with unique variations on each document, the majority of the images formed on said documents presenting a physical anti-copy characteristic satisfying a pre-defined criterion such that said characteristic of the majority of the copies that can be produced based on these images do not satisfy said predefined criterion; a means of characterizing said variations using frequency analysis for each document of said plurality of documents, to determine a unique imprint representative of a frequency spectrum of the image marked on said document; and a mans of memorizing said unique imprint.
 12. The device according to claim 11, wherein the means of generating an image is configured to generate an image designed to be degraded during copying of a document obtained by the means of marking.
 13. The device according to claim 11, wherein the means of characterizing variations uses encryption with a secret key.
 14. The device according to claim 11, wherein the means of memorizing is configured to store the imprint in a database.
 15. The device according to claim 11, wherein the means of memorizing is configured to mark the document with a mark representing the imprint affixed individually on the document.
 16. The device according to claim 15, wherein said mark representing the imprint is encrypted.
 17. The device according to claim 16, wherein the encryption utilized to encrypt the imprint uses a public key. 